In most common sulfur recovery plants, tails gas from a sulfur recovery unit is cooled by countercurrent heat exchange with a process liquid (typically water) in a DCC tower that includes a heat exchange section having a packed bed, a grid, trays, angle irons, or combination thereof to improve heat transfer. Hot gas rising through the packed bed cools down as it contacts the descending cooler process liquid, which is heated in turn by the hot gas. As the gas cools, condensable vapor condenses out of the gas, which further heats the process fluid. The heated process liquid is then collected at the bottom of the bed by a collector device, usually a chimney tray. From there, the heated process liquid is pumped through external heat exchangers that cool the process liquid, which is then returned to the top of the tower.
It is generally preferred to minimize the temperature of the gas leaving the top of the tower as hydrogen sulfide is more effectively absorbed in a downstream amine unit, and as the cooler treated gas comprises less water that would otherwise dilute the amine solvent. In most known DCC processes, a narrow approach (typically 5° F. or less) is taken between the outlet gas temperature of the DCC and the incoming cooled process liquid, which typically requires the temperature of the cooled returned low process liquid to be between about 95-115° F. To achieve such cooling of the process liquid, pumparound exchangers usually use cooling media such as water or ambient air. Where there is a small cost differential between water cooling and air cooling, and water is readily available, water cooling is often preferred. On the other hand, where air cooling is cheaper or water is not readily available, air it is typically preferred as the coolant. However, air cooling is often insufficient to provide the desired tower outlet gas temperature, especially where the DCC is located in a hot climate zone. In such cases, an air cooler can be operated serially with a water cooler such that the air cooler is used to remove the heat from the process liquid at the warmer temperature and that the water cooler cools the process liquid to the desired temperature. Unfortunately, as availability of water in sufficient quantities for water cooling is often problematic in hot climate zones, coolers using external refrigerants (e.g., propane) are often needed.
Where suitable quantities of water for water cooling are not available or expensive, or where an external refrigerant is used, there is an incentive to maximize air cooling of the process liquid. For example, the size and the duty of the air cooler can be increased. However, upsizing is typically limited by the approach temperature difference between the air and the process liquid. Most DCCs need to maintain a 15-20° F. approach between the air temperature and the temperature of process water leaving the cooler. Reducing the approach beyond this range leads to rapidly escalating air cooler size and cost.
Alternatively, the pumparound circulation rate can be reduced, which leads to higher process liquid temperatures leaving the packed bed. At these hotter temperatures, the heat can be removed from the process liquid by air rather than refrigerant. There is a strong dependence between the process water outlet temperature leaving the bed and the fraction of heat duty picked up by the air cooler. The hotter the bed outlet temperature, the higher the fraction of heat picked up by air cooling. However, as the pumparound circulation rate is reduced, the temperature of the process liquid rises throughout the bed and quickly approaches the gas temperature. As a result of the so reduced temperature difference, heat exchange in the bed is reduced and typically increases the bed height requirements. With large diameter towers, this additional bed height is expensive.
Worse yet, as the temperature difference between the water and gas in the packed bed diminishes, the bed becomes more prone to gas and/or liquid maldistribution, thereby leading to unreliable heat transfer performance. Maldistribution generates radial variations in liquid and gas temperature in the bed. When the temperature difference between gas and liquid is small, even small radial temperature variations can bring heat transfer to a halt or at least cause a major reduction of heat transfer rates. The high sensitivity to maldistribution reduces tower reliability and renders designs very unforgiving, and it is therefore generally preferred to keep the difference between the water leaving the bed and the gas entering (the “inlet approach”) to no less than 10° F. However, even such inlet approach requires close attention to liquid and gas distribution, and often results in substantial capital requirements for suitable distribution equipment. Even so, lack of robustness of operation may cause significant performance problems in case of corrosion, fouling, or other abnormal circumstances.
Therefore, while various systems and methods of cooling process liquids in sulfur recovery plant direct contact condensers are known in the art, all or almost all of them suffer from one or more disadvantages, especially where the direct contact condenser is located in a relatively hot and arid climate. Thus, there is still a need for improved direct contact condensers in sulfur recovery plants.